Cut the bread into 1-inch cubes and leave out over night on a cookie sheet. Preheat the oven to 350 degrees F. Melt the butter in a 9 by 13-inch casserole dish.
Place the bread in a large bowl with apple slices.
Combine the heavy cream, milk, Rumchata, eggs, brown sugar, vanilla, cinnamon, and salt, in a large bowl. Whisk to mix. Pour the cream mixture over the bread, and stir to combine. Allow the mixture to sit at room temperature for 30 to 45 minutes.
Transfer the bread mixture to the casserole dish and bake until the center of the bread pudding is set, 50 to 60 minutes. Serve warm with warm Fireball Whisky Sauce.
In a 1-quart saucepan set over medium heat, combine the cream, milk, and sugar. Place the cornstarch and 1/4 cup of the whiskey in a small mixing bowl and whisk to blend and make a slurry. Pour the slurry into the cream mixture and bring to a boil. Once the sauce begins to boil, reduce the heat to a gentle simmer and cook, stirring occasionally, for 5 minutes. Remove the sauce from the heat, add the salt, and stir in the butter and the remaining 1/2 cup of whiskey. Serve warm.